This invention relates to a process for the production of synthetic rubber latices having a high solids content and a low viscosity using low temperature redox emulsion polymerization with subsequent agglomeration of the latex particles with oxidized polyethylene oxides.
It is known that the latex particles must be relatively large and their particle size distribution must be relatively broad in order to obtain synthetic latices having a high solids content and a low viscosity.
Two different types of methods are known to obtain synthetic rubber latices having the required particle size and particle size distribution.
In accordance with the first type of process, the required particle size properties are achieved during the course of the polymerization. For this purpose, polymerization systems are utilized wherein the ratio of monomer to water is adjusted to about 0.8 to 1.5, i.e., a "high solids" ratio is employed. The polymerization reactions are started with a relatively low amount of emulsifier, i.e., up to 2 parts by weight, based on the amount of the monomers to be polymerized. These polymerization systems contain, in addition to an increased content of dispersing agent, such as for example a condensation product of formaldehyde and naphthalenesulfonic acid, still other chemical agents for increasing the particle size such as alkali metal salts in a concentration of &gt;70 millimoles/liter; (R. W. Brown and L. H. Howland, "Growth and Agglomeration of Particles in Low-temperature GRS Type Latex", Rubber World, July 55, p. 47) or also water-soluble salts of bivalent cations in a concentration of 0.01-1 mmol/l., in combination with water-soluble, higher-molecular weight agglomerating agents in minor concentrations according to DT-OS 2,165,410. It is also possible to cause the latex particles to agglomerate in systems of higher hydrocarbon/H.sub.2 O ratios by mechanical means with the use of thorough agitation at lower electrolyte concentrations of &lt;30 mmol./l. (U.S. Pat. No. 3,080,334). However, both kinds of polymerizations require time periods of &gt;12 hours in order to achieve a conversion of 60-70%. Moreover, the reactors must be cleaned frequently. This is a grave disadvantage, for example, in view of modern polymerization plants with reactor sizes of 200 m.sup.3, since expensive cleaning procedures become necessary. Moreover, it is difficult to produce latices having a uniform quality with the use of these processes.
On account of the aforedescribed disadvantages, polymerization systems have been utilized in more recent times with advantage in this technology which pertain to a second process type, wherein a ratio of monomer to water of 0.55 to 0.80, i.e., a "low solids" ratio, is employed. This makes it possible to conduct the polymerization in relatively short periods of time of &lt;10 hours. The latex particle size and/or distribution are adjusted only after the polymerization in a separate process step by the use of mechanical means, such as pressure homogenizers or chill rolls, as well as by chemical means, such as, for example, by addition of oxidized polyethylene oxides. A disadvantage of these processes is the increased energy consumption required to concentrate such latices to high solids contents. Larger devices are required for the concentration step. The latex must furthermore be recirculated relatively frequently during the concentration in the forced circulation evaporator by way of heat exchangers. This produces increased manufacturing costs in this process stage.